feat: 添加三元运算符支持，更新解析器和解释器以处理条件表达式

src/interpreter/interpreter.rs

@@ -361,6 +361,14 @@ impl Interpreter {
                     }
                 }
             }
+            Expr::Ternary { condition, then_branch, else_branch } => {
+                let cond = self.evaluate(*condition)?;
+                if self.is_truthy(&cond) {
+                    self.evaluate(*then_branch)
+                } else {
+                    self.evaluate(*else_branch)
+                }
+            }
             Expr::Call { callee, arguments } => {
                 let func = self.evaluate(*callee)?;
                 let mut args = Vec::new();
@@ -730,6 +738,57 @@ mod tests {
         }
     }
 
+    // =========================================================================
+    // Ternary
+    // =========================================================================
+
+    #[test]
+    fn eval_ternary_true_condition() {
+        assert_num(&eval_expr("true ? 1 : 2"), 1.0);
+    }
+
+    #[test]
+    fn eval_ternary_false_condition() {
+        assert_num(&eval_expr("false ? 1 : 2"), 2.0);
+    }
+
+    #[test]
+    fn eval_ternary_truthy_condition() {
+        // Non-nil, non-false values are truthy
+        assert_num(&eval_expr("1 ? 42 : 99"), 42.0);
+    }
+
+    #[test]
+    fn eval_ternary_nil_condition() {
+        assert_num(&eval_expr("nil ? 1 : 2"), 2.0);
+    }
+
+    #[test]
+    fn eval_ternary_nested() {
+        let interp = run("let x = 5; let r = x > 3 ? (x > 10 ? 100 : 50) : 0;");
+        assert_num(&get_var(&interp, "r"), 50.0);
+    }
+
+    #[test]
+    fn eval_ternary_with_computed_branches() {
+        let interp = run("let x = 10; let r = x > 5 ? x * 2 : x / 2;");
+        assert_num(&get_var(&interp, "r"), 20.0);
+    }
+
+    #[test]
+    fn eval_ternary_short_circuit_then() {
+        // When condition is false, then_branch is not evaluated
+        let interp = run("let x = 0; let r = false ? (x = 999) : (x = 42);");
+        assert_num(&get_var(&interp, "x"), 42.0);
+    }
+
+    #[test]
+    fn eval_ternary_short_circuit_else() {
+        // When condition is true, else_branch is not evaluated
+        let interp = run("let x = 0; let r = true ? (x = 42) : (x = 999);");
+        assert_num(&get_var(&interp, "x"), 42.0);
+    }
+
     // =========================================================================
     // Unary
     // =========================================================================